Field of the Invention
The present invention relates to an estimation and compensation technology for overcoming RF imperfections, particularly to a joint estimation and compensation method of RF imperfections, such as IQ imbalance, shaping filter imbalance, DC offset, multipath channel and carrier offset, in an uplink system.
Description of the Related Art
Succeeding to HSPA (High Speed Packet Access), LTE (Long Term Evolution) is a wireless access standard proposed by 3GPP (3rd Generation Partnership Project) to achieve further higher transmission efficiency. ITU (International Telecommunication Union) has formally defined LTE as 4G in 2010. LTE can enhance network transmission capacity and speed and support higher requirement of wireless communication.
According to the LTE specification proposed by 3GPP, an uplink system uses SC-FDMA (Single Carrier Frequency Division Multiple Access) signals, and a downlink system uses OFDMA (Orthogonal Frequency Division Multiple Access) signals. The present invention is primarily involved with the technology of LTE uplink systems. Users of mobile devices often care about power consumption and price. Higher PAPR (Peak to Average Power Ratio) will cause high price and high power consumption. Therefore, the LTE standard adopts the low power consumption SC-FDMA technology for uplink systems.
In an LTE uplink system, RF impairments are likely to occur in the DAC/RF device of the transmitter or receiver, such as IQ (In-phase/Quadrature-phase) imbalance, shaping filter imbalance, and DC offset. In other words, the baseband transmitter generates complex baseband signals; the baseband signals are divided into real parts (I signals) and image parts (Q signals); the I signals and Q signals are processed by DAC and respectively multiplied by a cosine wave and a sine wave, which have an identical amplitude and an identical frequency but have a phase difference of exactly 90 degrees; then, the signals are carried by a radio frequency and transmitted. On receiving the signals, the receiver down-converts and demodulates the signals. However, mismatches are likely to occur between the oscillators respectively generating sinusoidal waves in the transmitter and the receiver, including magnitude mismatches, phase mismatches, and oscillation frequency mismatches. The magnitude mismatch is called the magnitude imbalance. While the phases lack complete orthogonality, the phenomenon is called the phase imbalance. The two imbalances are called the IQ imbalance in combination. On the other hand, the transmitter and receiver must use a shaping filter to decrease the bandwidth of signals so as to meet the demand of the system and reduce ISI (Inter-Symbol Interference). The Nyquist filter and the SRRC (Square Root Raise Cosine) filter are often used to shape the signals of the transmitter and the receiver. While the transmitter and the receiver respectively adopt different shaping filters, shaping filter imbalance may take place there between. The cheaper direct-conversion architecture is often used to reduce the cost. In such a case, a portion of the power of the local oscillator leaks to RF signals and mixes with the transmitted signals, which will cause IQ DC offset in the transmitter.
Besides, multipath propagation usually occurs in a wireless communication system because of refraction, diffraction or scattering in an indoor or outdoor environment. In such a case, the receiver will receive two or more signals from different paths in different delay time, which will cause ISI and degrade the performance. Further, while up- or down-conversion is undertaken between the transmitter and the receiver, incomplete synchronization of the oscillators will cause frequency offset. Furthermore, Doppler shift occurring in high speed movement will lead to CFO (Carrier Frequency Offset). CFO will seriously affect the SC-FDMA- or OFDMA-based system, not only interfering with wireless communication but also leading to ICI (Inter-Carrier Interference).
The signal imbalances, such as IQ imbalance, shaping filter imbalance, DC offset, multipath channel, and CFO, will lead to RF mismatch in an LTE uplink system. At present, most of the related technologies undertake estimation and compensation in the frequency domain. However, they do not apply to LTE uplink systems. So far, neither a joint estimation nor a joint compensation method has been published, not to mention a time-domain joint estimation and compensation method considering various RF attenuations.
Accordingly, the present invention proposes a joint estimation and compensation method of RF imperfection in an uplink system to overcome all the imperfections in LTE uplink systems.